Thermionic amplifier



Patented Nov. 25', 1947 @situation finvention, relatesf.:todi-amplifiers 4using: thermionic `va1ves;zand has vfori'-itsnobject auto-[ maticallygto s maintain.;l constant .l the gain Tof r""fa s: :2

valve.` lciespitez` `wmitimi y infizthewsourcefof ffsuchf voltage. For such f purpose :the'liinvention mak usel of afthermistor or thermistors.S1l 1 l, v'Ihermistors `have 1 been sin.: use for sox'ne" f'years* andare characterised by? a.,temperature` coe'ifi-h. -'t

cient of resistance whichzmayz be .eitherfpositix'fe' or. negativedand which is `moreover"many :times thecorresponding v,coeicient forfa purefmetal such as lcoppers- This'gfproperty rendersflfthe'rf'y mistqrs particularly'l-suitablef'or a variety .ofy A'sp cial -applicationsfin' electric ,circuits-U VariousdiiferentI materials aref'available .fori the ,resistance element:11315:;` :ai thermistor, these? various materials having- ,diIferentc-properties iny other respects; as `one Lex-ample; fa' resistancema terialvhavinga high negativedzemperature fcceicient of kresistance 1 comprises amixture' vr:of

manganese oxide ,and nickel oxide;l winner withiI out the laddition of y certainsother metallic oxides; f

the 'xnixturebeinggfsuitablyg heatz-treated. il, .ir

'Iherniisto'rs have beenlemployed l-.ingtwo'i difzf ferent` forms: ,v (la), known as a. directly heated-:1 thermistor.l and comprising:'al` ,resistance element of the thermally sensitivefresistancematerialproL-'l vided withv suitable le; 1.doutl conductors;` or ter'- minals,`` and (b) known fas f an;indirectly. heated?? tl'lerrnistoncomprising.thegelementf` (al provided y30 in .addition .wy/ith. a l locating;coilg,y electrically fines i sulated from the element. A `directly heatedu.` thermistor iS primarilyintended. to `be controlled by the current ywhich fiowivsthroughlitandwvhich varies the yternperatui'e 'and also the "resistancelf35 acerdingly :Suelaa.;tilminister willv aises-be :af-f. fected `-byf-tlie temperature,v ofi 1 its; :surroundings :if and may iherfore t be. usedforf thermostatic con'- trolV and like purposes y or., Nvithoutvfdirect" I heating by the; current iiowing, through' it. #Anm-'Jy f 40 directly heated'y thermistor lis chieihiy designed" to i" be heated Vby av controlling; lcurrent; which Aflows 1': through the heating coiland v7i1-ichwi1lusually, ,e bui? hot 1160955211115', ,be 'different iromulth'e current; whichlliiows thioughthe-.res`i.sta1;ic eg element,r butA '45 thisA t'ypelof thermistor inay alsobe subjected ,to f. either .or Y both lof fthe types of;contro1 'applicable` e to; a directly.; heated fihermister M0116 f detailed- ,ini(nfmationI on l .the fpropertiesi-i' 0f -th'e'rmistors wllfbe ,foundinensnticle byri'Gi-o L. Pearson inthe Bell Laboratories Record Dec.:

ci 'lie-iii) of Jagtvalve :against variatioriiin the'fvheating-'Mcurand cathode ofthe v'alve l"as to vary thefbias on' um;l graafofwthelvaiveitofcompensate" f er jcitfmges'l in said source. f 'i "The 'L'ntui vof thel "invnfiiwll be L' 'better understod fro'nif the following description taken? in vcom'uri-ation with the-'Lacco 'ngdrawings in Whichff lf'lwz; 1f," i 1,1513' ,1;

.'FigsVlZ andi-'S'showcircur irjcoritrollingfl the gridbiasI of ai therrnonic valve' to! compensate" for'v'artios in the vlta'ge of thehigh'tensio source: 1: #1 e.. l Fig. 4 shows a circuit Iorf'stabilisirigfthe gai'xif rentgfor` the cathode.' Ik 1 l Referring. to :the drawing and more paairtic-f` ularly f to Fig. r 1, a fthermionic' valve i.` I fisi shown-f for convenience ofillustratlon aff-triode. 'The il power;y supplies are conventionally` shown as bat"-v teries:jbutl =theyi-may beffprovid'edf in anyfwell knowny ffway.' The loadwL ."is.|` connected between the 1 pQsitive: terminal. of:v thebattery HT and the f anode. Asutable bias isapplietiizov the grid from Y a yportion' of,=. thefhigh tensionbatteryfI andwthisf i encepdevelopedacross a directly heated thermistorv@ 3 I lhwnega` negative-1. temperature coefficient n off` vlia'ge Qf the batteryffI-IT, rises; :the resistance.

of therrnistor E3 g fallsfand thus the i potential` diffu ference across it. rIfhe jbias Y onV the grid then becomes more ,negative, If :the` potential-v. of: 4the-1 n' battery-HT .falls l a reverseaction takes. place: and

coinpensfationyyfillnot becomplete. t

ging,

across kthe, sisting o l v vrrangement should@ preferablyA vbe .v such ,i that hec han geinr` vvoltage,vr on the y grid oft-the:vv

y u,riseqv ue1'it-` upon f a change in `voltage 'vof Y ,tliebattery I-I'I?v is `1/g,u of. the latter, where wis they: v tithe `valve. l Otherwise, the

v hows an arrangement'. similar" to that oflll'igliylfbut using anfindirectlyheated'thermistor ,t TIIIL ,this 9=h11heating com of the then, si mistor` is; connectedv in r`series .with resistance :5f batterylllllwhilst a; potentiometer con-f y lhe resist zatncev R-o thermistor 'T' andi-y a ,constant resistance f .2 `is connetedlacross `the battery HT and grid bias battery GB. Resistance l and thermistor I are shunted by another resistance 1. A rise in voltage oi' the batterycauses a decrease in resistance 6 and hence an increase inthe potential diiierence across resistance 'l applied to resistance I and thermistor l. This means an increase of current in thermistor 3 .and a decrease in resistance and hence a decrease in the potential diierence opposing the battery GB and an increase in the negative grid bias. The reverse action takes place on a. fall in the -voltage of the battery.

In a similar .manner a directly heated thermis- Fig. 2 or in series therewithl and has the effect of increasing the effect in the heating coil r-of a change in the battery voltage and thus increases the eiiective .change in grid bias with a given change of battery voltage as compared with the circuit 'of Fig. 2.

vT'hese arrangements may be used to control the grid bias'of `a plurality of valves.l The grid of each -valve may -in such case be connected through an individual grid leak resistance I to aJ point on a potentiometer including a, thermistor y 3 as in Fig. 1 or Fig. 3 or the resistance R of an indirectly heated thermistor as in Fig. 2,. these points being diierent for valves requiring differ ent amounts of grid bias.

The variation of resistance of a thermistor with current passing through it may also be used to change the grid bias to compensate for changes' in the voltage across the cathode heater. Fig. 4 shows a circuit for this purpose. The heater rr of an indirectly heated thermistor T1 -is connected tor may be connected in place of'resistance 5 in 'mid-pointfof the straightest and/or steepest portion of the characteristic. The heating power should preferably be as large as possible compatible with this requirement in order to reduce lthe eiiect of changes in the ambient temperature.

What is claimed is: 1/ In a device for maintaining the gain oi a thermionic valve ampliner constant despite variations in -voltage o! a source supplying said ampliier, a thermistor including heater and resistance elements, means for heating said resistance element proportionately to the voltage of saidv source and independently of any signal current passing through said amplifier, said lmeans comprlsing an impedance connected in series with said heater and connections placing said impedance and said heater in series across said source, a source of negative bias potential, a connection from one end of said resistance ele-. ment'to said source of negative bias potential, a

grid resistor, a. connection from the 'other end in parallel with the heater of the valve I to the y source HS. The grid bias is supplied from battery GB through resistance 4 from a potentiomcurrent causes an increase in resistance R1 and a reduction in the biasl on the grid of valve I, to compensate for the fall in electron emission from the cathode. f v

Part only of the cathode heater voltage may be used for -the thermistor heater if desired. Con- I `eter comprising the resistance Ri of the thermistor T1 and resistance 8. A fall in the heater densers C; C may be inserted in series with the thermistor heater r1 so as to reduce the voltage ac ross the cathodeheater forthis purpose.

In the above description it has been assumed all through that the thermistors have a negative temperature coeiilcient of resistance. If desired, however, thermistors with apositive temperature vt() the circuits. Thus, in Fig. 1, if the thermistor 3 be interchanged with resistance 5, the same result will be obtained when the thermistor has a,

positive temperature coeillcient. For the same purpose in Fig. 2 the resistance element R of the thermistor `T should be 'interdhanged with resistance 2; and in Fig. 3, thermistor 3 and resistance 5 should be interchanged, and also ther-` mistor 6 with resistance 1. In Fig. 4 v the resistance element R1 of the thermistor T1 shouldbe lnterchanged with resistance 8. It will be evident, also, that where the circuit contains two thermistors (such as in the case of Fig. 3), they can have temperature coelcients of opposite sign if desired, and it will be evident fromthe above explanation how the circuits should `be arranged in each case. l

As ythe thermistors will be affected to sonic extent by changes in the ambient temperature, it f may be necessary to provide means in any well .coeiilcient may be used with small modifications of said resistance element to said grid resistor, a connection from said grid resistor to the control 4grid of saidn valve, a second resistance external to said thermistor and connected at one rend to the same end of said resistance element to which said grid resistor is connected, and a connection from the. other end of said second resistance to a point positive with respect to said controlvgrid, the relative values .of said thermistor resistancey element and said second resistance being selected so that said control grid vis at all times maintained negative with respect to the cathode .of said valve, whereby said thermistor functions to vary the bias on the gridof said valve pensate for changes in said source.

2. Device according to claim 1, in which said thermistor is arranged to have a, negative temperature coefcient, whereby decrease of the voltage of said source decreases the negative bias on said control grid.

3.- Device according to claim 1, in which said amplifier has a cathode and in which said thermistoris of the indirectly heated type andsaid second resistance i's connected between said thermistor resistance element and'y said cathode.

4. In a device for maintaining the gain of ra thermionic valve amplifier constant despite vari- -v ations in voltage of a source supplying said amplier, said amplifier 'containing a cathode and control grid, a thermistor including a heater and a resistance element having a-positive temperature coecient, means for heating said 'resist-"j ance element proportionately to the voltage of said source and independently of any signal current passing through said amplifier, said means comprising an impedance connected-in series with the heater of said thermistor and connections placing impedance and heater in s eries across said source, a grid resistor, a. connection from one end of the resistance element to a point positive with respect to said control grid, a connection l from the other end of said resistance element' to said grid resistor, a connection from said grid resistor to the control grid of said valve. a second resistance external to said thermistor and connected at one end to thesame end oi said -reor reducing this A to comp--v sistance element to which said grid resistor is connected. a source of negative bias potential and a connection from the other end of said second resistance to said source of negative bias potential, the relative values of said thermistor and said second resistance being selected so said control grid is at all times maintained negative with respect to said cathode.

5. In a device for maintaining the gain of a thermionic valve amplier constant despite variations in voltage of a source supplying said ampliiler, said amplier containing a cathode and a control grid, a thermistor of the directly heated type. means for heating said thermistor proportionately to the voltage of said source and independently of any signal current passing through said amplier, said means comprising an impedance and a resistance in series therewith, said impedance and resistance forming a single element connected in series with said thermistor and connections placing said single element and said thermistor in series across said source, said source consisting of an anode source and a negative bias source connected in series, a connection from one end of said thermistor to said source of negative bias potential, a grid resistor, a connection from the other end of said thermistor to said grid resistor and a connection from said grid resistor to the control grid of said valve, the relative resistance values of said thermistor and said single element being selected so that said control grid is at allv times maintained negative with respect to the cathode of said valve.

6. In a device for maintaining the gain of a thermionic valve ampliiler constant despite variations in voltage of an A. C. source heating the cathode of said amplier, a thermistor including a heater and a resistance element, means for heating said thermistor proportionately to the voltage or said source, said means comprising condenser means connected in series with said heater, a source of negative bias potential, a connection from one end of said resistance element to said source of negative bias potential, a grid resistor, a connection from the other end of said resistance element to said grid resistor, a connection from the grid resistor to the control grid of said valve, a, second resistance external to the thermistor and connected at one end to the same end of said resistance element to which said grid resistor is connected and a connection from the other end of said second resistance to a point positive with respect to said control grid, the relative values of said resistance element and said second resistance being selected so that said control grid is at all times maintained negative with respect to saidcathode.

7. In a device for maintaining lthe gain of a thermionic valve amplifier constant despite variation in voltage of a source supplying said ampliiler, said amplier containing a cathode and control grid, a thermistor of the directly heated type, means for heating said thermistor proporand its series resistance, a grid resistor, a connection from a point between said second thermistor and its series resistance to said grid resistor, a connection from said grid resistor to the control grid of said valve, and the relative resistance value of said thermistors and said rcsistances being selected so that said control grid is at all times maintained negative with respect to said cathode.

8. In a device for maintaining the gain of a thermionic valve ampliiier constant despite variation in voltage of sources supplying said ampliiler, said source comprising an anode source and a negative bias source, said amplifier containing a cathode and a, control grid, thermistor means connected with and responsive proportionately to the voltage of said sources and independently of any signal current passed through said signal amplifier, said thermistor means comprising two thermistor elements, a resistance connected in series with each thermistor element, connections between said thermistor means and resistances with said sources, connections between one thermistor element and the other element whereby the first aiects the response of the second. a. grid resistor, a connection from a point between a thermistor element and the resistance in series therewith to said grid resistor. a connection from the other end of said grid resistor to the control grid of said valve, the relative resistance values of the elements of said thermistor means and said resistances being selected so that said control grid is at all times maintained negative with respect to said cathode.

9. In a device for maintaining the gain oi.' a thermionic valve amplifier constant despite variation in voltage of sources supplying said amplifien said ampliiier containing a cathode and a control grid, thermistor means connected with and responsive proportionately to the voltage ot said sources and independently of 'any signal current passed through said signal amplifier, said thermistor means comprising two thermistor elements, an impedance connected in series with each thermistor element, one of the thermistor elements directly controlling the control grid bias and the impedance in series therewith being a resistance, connections between said thermistor means and said sources, connections between one thermistor element and the other element whereby the first affects the response of the second, a grid resistor. a connection from e. point between a thermistor element and the resistance in series therewith to said grid resistor, a connection from the other end of said grid resistor to the control grid of said valve, the relative resistance values of the elements of said thermistor means and said impedances being selected so that said control grid is at all times maintained negative with respect to said cathode.

PRAFUILA KUMAR CHATI'ERJEA. CHARLES THOMAS SCULLY.

REFERENCES CUED The following references are oi' record in the file of this patent:

A UNITED STATES PATENTS Number 

